FIG. 1 illustrates a conventional document scanner. In the conventional scanner, a light source 11 is used to illuminate a document 3 having an image thereon. The conventional document scanner also includes a glass platen 5 upon which the document 3 rests and a platen cover 1. FIG. 1 also shows the angle of reflection center line 9 for the conventional document scanner.
To scan the image on the document, the light source 11 illuminates the document 3 through the glass platen 5 such that the light reflected from the document 3 passes through on optical lens system 7. The optical lens system 7 directs the reflected light to either a photosensitive recording medium, a CCD sensor, or a full width array sensor. If the reflected light is directed to a photosensitive recording medium, a latent image of the document 3 is developed thereon and is subsequently transferred to a copy substrate. On the other hand, if the reflected light is directed to a CCD sensor or a full width array sensor, the light reflected from the document 3 is converted into electronic signals forming image data which electronically represent the document 3.
To provide a full scanning of the document 3, either the document 3 is moved relative to the light source 11 and the components which receive the reflected light, or the light source 11 and the components receiving the reflected light are moved relative to the document 3.
FIG. 2 illustrates, in more detail, the light source 11 for a conventional document scanner. This conventional light source includes a fluorescent lamp 111 which produces the light coming from the light source 11. Attached to either end of the fluorescent lamp 111 are lamp bases 110 which include electrical pins 118. These electrical pins provide an electrical conduit for the fluorescent lamp so that the fluorescent lamp can receive the proper electrical power. These pins 118 also provide mechanical support by holding the fluorescent lamp 111 securely in place.
For the fluorescent lamp to be fully functional and secure, the pins 118 are placed into fluorescent lamp holders 117 which provide the mechanical support for the fluorescent lamp as well as the electrical terminals which provide the electrical power to the fluorescent lamp. The fluorescent lamp holders 117 are each connected to a pair of electrical leads 116 which are in turn connected to a power source.
The fluorescent lamp 111 is also substantially covered by a heater blanket 112 which includes a heater element 113. The heater blanket 112 may include a small slit or be transparent to allow the light produced by the fluorescent 111 to pass through the heater blanket 112 and illuminate the document 3. The heater blanket 112 is provided to prevent undesirable cold spots within the fluorescent lamp and to enable the fluorescent lamp to produce a more stabilized light.
The heating element 113 is connected to a power source through contacts 114 and electrical leads 115. Thus, to properly assemble a conventional light source in a conventional document scanner, the fluorescent lamp 111 is placed in the fluorescent lamp holders 117 and the leads 115 are soldered to the heating element at contacts 114 located on the heater blanket 112.
Utilizing such a conventional light source as described above, with respect to FIGS. 1 and 2, the illumination of a document in a uniform manner becomes problematic. More specifically, in document illumination with a fluorescent lamp, the uniformity of document illumination in the axial direction depends on the length of the lamp. By extending the lamp well beyond the edge of the document, uniformity can be improved but his forces the size of the machine to grow in many cases.
For example, the illumination on a plane close to a fluorescent lamp is approximately uniform near the center and falls off toward the ends. The exact rate of decrease is dependent on the physical construction of the lamp, that is the diameter, electrode placement, filament size and shape, etc. In document illumination applications such as copier machines, it is common to extend the lamp well beyond the edge of the document to minimize the effect of the non-uniformity on copy quality. In electronic scanners, where there is often some electronic means of correcting for non-uniformity, it is still helpful to reduce the amount of falloff, particularly when other sources of non-uniformity are present. An example of such other sources is the relative illumination falloff due to the lens, commonly referred to as cos.sup.4 .theta. falloff.
Several methods exist for reducing the end falloff. Light/lens reprographic machines typically use a butterfly slit, wider at the ends than the center to allow a longer exposure time as the image is scanned. End reflectors have been used to create a virtual image of the lamp, making the lamp appear to be longer. Light/lens reprographic machines and electronic scanners have used relative illumination filters and blockers in the imaging path to change the apparent shape or transmittance of the lens depending on axial position. Such blocking features have included variable coverage halftone patterns on the lamp to reduce the illumination in the center.
However, in the document scanner environment, such solutions may not readily solve the problem. Such scanners image a narrow line, typically 0.06 mm, so methods involving slits etc. to vary the exposure tile along the line would require unrealistic precision. Using a variable width slit directly on the lamp is possible. With aperture lamps normally used for document illumination, however, this will have the undesirable effect of changing the transverse illumination profile, and so the positional tolerances of illuminator components.
Therefore, it is desirable to provide profile correction without adjusting the slit's width or other dimensions of the lamp. More particularly, it is preferred to vary the "apparent" slit length along the lamp by using blocking features that are perpendicular to the lamp's axis.